This invention relates to an optical communication network unit and, more particularly, to an optical signal converting apparatus and an optical signal receiving apparatus for use in the optical communication network unit.
As is well known in the art, it is possible for optical communication to make a transmission capacity for a single optical transmission path larger by carrying out a wavelength multiplexing on a plurality of optical signals. However, it is impossible to multiplex a plurality of optical signals each of which has the same wavelength into a multiplexed optical signal. This is because it is impossible to demultiplex or separate the multiplexed optical signal having a single wavelength into a plurality of multiplexed or separated optical signals. For this purpose, it is necessary to carry out wavelength conversion in a case of multiplexing a plurality of optical signals each of which has the same wavelength.
A wavelength conversion device converts an input optical signal having an input wavelength into an output optical signal having an output wavelength. Among wavelength conversion devices, wavelength conversion devices for digital modulated optical signals are classified into a noninverting wavelength conversion device and an inversion wavelength conversion device. The noninverting wavelength conversion device is a conversion device where the output optical signal has bit logic which is equal to that of the input optical signal. On the other hand, the inverting wavelength conversion device is a conversion device where the output optical signal has bit logic which is obtained by inverting bit logic of the input optical signal. If desired, reference should be made to any relevant publication that is most readily available. An example of such publications is an article contributed by J. Zhou et al. to IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 6, NO. 8, AUGUST 1994, pages 984-987, under the title of "Four-Wave Mixing Wavelength Conversion Efficiency in Semiconductor Traveling-Wave Amplifiers Measured to 65 nm of Wavelength Shift."
As an example of noninverting wavelength conversion, a wavelength conversion using four-wave mixing in a semiconductor laser is cited. As an instance of inverting wavelength conversion, a wavelength conversion device using mutual gain modulation in a semiconductor optical amplifier is cited. An instance of the inverting wavelength conversion device is disclosed in an article which is contributed by C. Joergensen et al. to ELECTRONICS LETTERS, Vol. 32, No.4 (Feb. 15, 1996), pages 367-368, and which has a title of "40 Gbit/s All-Optical Wavelength Conversion by Semiconductor Optical Amplifiers."
As described above, inasmuch as there are the inverting wavelength conversion device and the noninverting wavelength conversion device, there are two types of optical signals which are transmitted in an optical communication network system. That is, a first type of optical signal is an optical signal whose bit logic is inverted while a second type of optical signal is an optical signal whose bit logic is not inverted. The former is called an inverted optical signal and the latter is referred to as a noninverted optical signal. Accordingly, an optical receiving unit in the communication network system may be supplied with, as an input optical signal, either the inverted optical signal or the noninverted optical signal.
In the manner which will later be described in conjunction with FIGS. 1 and 2, each of a conventional optical signal receiving apparatus and a conventional optical signal converting apparatus comprises an optical branching unit and an optical switch. As a result, each of the conventional optical signal receiving apparatus and the conventional optical signal converting apparatus is disadvantageous in that it results in having a larger mounted space and in increasing cost of product.